I suppose an actor on a stage doesn't _need_ an audience, either; he could applaud himself. Now there are certainly actors who act for the income (many of them are in Hollywood), and there are actors who act out of a desire to have others applaud them (Nero is said to have been one such). But I've seen plenty of people on stage who were there because they genuinely liked to entertain people, and receive the applause graciously; and for many of them, it doesn't go to build up their egos, rather they get enjoyment out of making other people happy.
I'm not saying God is an entertainer, but I also don't think he has an ego maniacal need for someone to glorify him, and he certainly doesn't do it for the money (since he owns everything he created already). But that leaves plenty of other possibilities. For instance, there are some things in the world that many people from across the religious-to-atheist spectrum would agree are admirable (in some sense): the universe, the Grand Canyon, Mt Rainier, a flower... And admiring them does us some good, like getting our mind off ourselves. At least from a Christian (and Jewish, and I think Islamic) perspective, admiring God is admiring the one who created such beauty. And I see nothing wrong with it.